Hitherto, various foamed materials have been industrially widely produced and sold. Typical examples of foamed materials include foamed materials of rubber having both the flexibility and sealing properties. Production of a foamed material of rubber usually includes a step of cross-linking and foaming a raw material rubber composition.
In the above-described foaming, a chemical decomposition type foaming agent, which generates a gas by decomposition due to heating, has been used. Known examples of various foaming agents include azodicarbonamide (ADCA), N,N′-dinitrosopentamethylenetetramine (DPT), and 4,4′-oxybis(benzenesulfonyl hydrazide) (OBSH) (refer to PTL 1 and PTL 2, for example).
However, the chemical decomposition type foaming agent has problems, such as, inhibition of cross-linking resulting from a foaming agent, and mold pollution and environmental pollution caused by a foaming agent residue. In particular, DPT is a substance which in itself has a risk of explosion or fire and which is suspected of having a possibility of becoming a mutagen and, therefore, has a drawback that the safety for a human body is poor. Consequently, development of an excellent foaming agent which does not have the above-described problems and which can be used as an alternative to the chemical decomposition type foaming agent has been desired.
Meanwhile, a method for manufacturing a foamed material of a thermoplastic elastomer by using water as a foaming agent has been known (refer to PTL 3 and PTL 4, for example). In these Patent Literatures, the subject of foaming is a thermoplastic elastomer, and a predetermined foam-molded material is prepared by pressing water serving as a foaming agent into an extruder.